Have you ever noticed warning signs posted along roads stating "beware of falling rocks"?
These signs let you know that you are in a hazard zone, an area where rockfall is common!
And shear strength is a function of cohesion (ability of particles to attract and hold each other together) and internal friction (friction between grains within a material).
Climatic conditions and softer rock types result in material that weather rapidly and erode easily. Slope stability is based on the interplay between two types of forces, driving forces and resisting forces.
The main resisting force is the material's shear strength. Mass movement occurs much more frequently on steep slopes than on shallow slopes. In the form of rivers and wave action, water erodes the base of slopes, removing support, which increases driving forces.
Water can also increase the driving force by loading, i.e., adding to the total mass that is subjected to the force of gravity.
Rockfall (free fall of rock) is an extremely rapid process and occurs without warning. Frost wedging is a process where water enters cracks in rocks, freezes, expands, and breaks the rock apart.
Frost wedging results in a fan-shaped pile of rock fragments at the base of the slope.
Chemical weathering (interaction of water with surface rock and soil) slowly weakens slope material (primarily rock), reducing its shear strength, therefore reducing resisting forces.